I don't know the answer but it certainly seems that in nature that could be part of the natural cycle. Time will tell for you so let us know how they turn out.
If you haven't kicked the lettuce out of the crisper yet you are not behind the eight ball. I have never had media freeze in the crisper.
On the other hand if it is one of those small fridges you might try a sheet of Styrofoam cut to fit under the freezer to moderate the temperature.
